A bumper assembly of an automobile is composed a bumper fascia, an energy absorber, a back beam, a stay, and other components, from the outermost side thereof.
Among these components, the stay is placed between the back beam and a car body to interconnect the same. Generally, fastening holes are provided corresponding to the back beam and the stay, respectively, and the back beam and the stay are fastened to each other through the fastening holes by fastening members, such as bolts and nuts, and the like.
When a vehicle collides at a certain speed limit, collision is absorbed through deformation of the energy absorber, deformation and destruction of the back beam, deformation of the stay, and the like. In this case, various patterns of stress are applied to the back beam. Stress can be classified into tensile stress, compression stress and shear stress according to directions of the stress, and different types of stress can appear at different sites on the back beam.
In order to achieve efficient absorption of collision, various shapes of the back beam have been studied and developed together with diversification of materials thereof.
Particularly, for the purpose of weight reduction, development of a back beam using a fiber-reinforced composite material formed by mixing reinforced fibers and plastic (synthetic resin) materials as main components has been conducted in recent years.
According to length, fibers can be classified into staple fibers, filament fibers and continuous fibers, and it is known that the back beam formed of the fiber-reinforced composite material exhibits better mechanical properties and provides better weight reduction effects with increasing length of the fibers.
The composite material is often formed using a composite material of staple fibers and continuous fibers and a composite material of filament fibers and continuous fibers simultaneously in order to use mechanical properties of the continuous fibers and formability of the composite material with the staple fibers or the filament fibers. The back beam of these composite materials may be manufactured by various methods such as injection molding, compression, RTM, welding, and the like.
However, it is known that the fiber-reinforced composite material exhibits a high energy absorption rate when receiving tensile stress, and a low energy absorption rate upon receiving compression or shear stress.
A prior art literature relevant to the present invention includes Korean Patent Publication No. 10-2005-0115485 (published on Dec. 8, 2005) entitled “Bolting structure of bumper stay.”